A polychromator including a light receiving element array in which a great number of light receiving elements are arrayed and a diffraction grating is often used for a measurement device that uses light dispersion such as a spectrophotometer, spectroscopic luminance meter, and the like (also referred to as a spectroscopic system). The diffraction grating may disperse the light to be measured (also referred to as target light to be measured) according to a wavelength. The light receiving array may acquire an electric signal corresponding to an intensity of the light for every wavelength by receiving the light dispersed by the diffraction grating in the great number of elements arrayed at a predetermined pitch (also referred to as an element pitch). A spectral distribution of the measuring target light is thereby obtained.
In such a polychromator, a case of adopting a plastic molded article for the main constituents such as the diffraction grating, housing, and the like is increasing for the purpose of reducing the manufacturing cost. However, the plastic molded article may cause thermal change and temporal change due to change in surrounding temperature, alleviation in internal stress after molding, and the like. Thus, a shift in wavelength (also referred to as a wavelength shift) may occur in the spectral distribution, which is the measurement result of the polychromator.
A spectral device, and a correction method of the spectral device capable of maintaining the initial measurement accuracy by correction corresponding to the wavelength shift have been proposed (Japanese Patent Application Laid-Open No. 2005-69784, etc.). In the technique of FIG. 4 of Japanese Patent Application Laid-Open No. 2005-69784, monochromatic light is entered to a slit and dispersed by the diffraction grating. In this case, a shift amount (a first image shift amount) from an initial position of an image (also referred to as a first dispersion image) formed on the light receiving element array by primary diffracted light of the slit-passing light is obtained. A shift amount (a second image shift amount) from an initial position of an image (also referred to as a second dispersion image) formed on the light receiving element array by a secondary diffracted light of the silt-passing light is also obtained. An image shift amount in which influences caused by the wavelength change of the monochromatic light are removed is then obtained based on the first and second image shift amounts, and then the image shift amount is converted to a wavelength shift amount.
In this technique, a phenomenon in which the first image shift amount and the second image shift amount are the same in the wavelength shift caused by the temporal and thermal changes of the polychromator, and the second image shift amount is twice the first image shift amount in the wavelength shift caused by the wavelength change of the monochromatic light is used. Specifically, the wavelength shift amount that occurs from the polychromator in which the influences caused by the wavelength change of the monochromatic light are removed can be acquired from the first and second image shift amounts. The correction of the spectral device corresponding to the wavelength shift amount then can be carried out.